That carb is too small for a 409 if it is truly for a 300 horse 327! You are are going to leave a lot of horse power on the table. A small block Chevy AFB will have the 4 1/8" air horn and flow about 500 cfm. You will need at least 600 cfm. That carb will have the 5 1/8" air horn and fit your dual snorkel air cleaner base.
I don't have the carb. # available as I write this, but I certainly can get it. I did research this number as to usage and it was used on the '64-'65 Corvette 327/300. I believe I also read from a flow chart that it flowed 575 cfm. I know ideally a 600 cfm would be best but I thought for time being, this would be close enough. The air horn does measure 4 1/8". I had this carb. rebuilt and on the dyno, it adjusted quite well. I'll be going to a large swap meet next month and keep my eyes open for another carb. Just wondering though, if an AFB air horn measures 5 1/8", is that a given that it flows at least 600 CFM?? Thank you, Carmine.